Conventionally, nickel oxide, and stabilized zirconia, whose crystal structure is stabilized, are used as the anode materials for solid oxide fuel cells. These materials are mixed in a process for forming an anode. In that case, the properties of the obtained anode largely depend on the properties of the material after being mixed.
For example, a nickel oxide in an anode is reduced from the nickel oxide state to a nickel metal by hydrogen gas, i.e., fuel, thereby to act as a conductor that efficiently conducts electrons produced in power generation, and as a decomposition catalyst for hydrogen gas. In this case, micronizing nickel and uniformalizing the distribution state thereof in the anode, namely, improving the nickel dispersibility, increases the specific surface area that can contribute as a catalyst. In addition, this improvement also leads to an increase in the number of boundaries of nickel, stabilized zirconia and pores, i.e., three-phase boundaries, which are known as reaction sites of fuels. Consequently, the electrochemical reaction is facilitated and the output characteristics are improved.
There is a known technique to improve such output characteristics in which nickel oxide and stabilized zirconia having a stabilized phase are pulverized and mixed using a media mill, such as a ball mill, to manufacture an anode material (Patent Literature 1).
However, in the anode material obtained by this technique, an impairment of nickel dispersibility may be observed due to the difference in specific gravity between the nickel oxide and the stabilized zirconia, and due to electrostatic aggregation of the particles. For example, particles having a submicron size of less than 1 μm cause a problem of remarkable electrostatic aggregation. In the case of particles having a particle diameter of about several microns, the sedimentation rate of the particles increase, and the nickel oxide and stabilized zirconia are separated from each other due to the difference in specific gravity, resulting in significant deterioration in the nickel dispersibility.
In view of these problems, Patent Literature 2 proposes a technique for improving nickel dispersibility, in which materials are mixed in an aqueous solution having a predetermined pH value so as to increase the electrostatic repulsive force of the particles of each material, thereby preventing aggregation among particles of the same material. In addition, in this technique, the electrostatic charge is set to be opposite for different materials so as to cause the particles of different materials to aggregate, thereby improving the nickel dispersibility. Generally, when oxide particles are dispersed in a polar solvent, such as water, the particle surfaces adsorb hydroxyl groups, and, as a result, the dispersion stability is enhanced by solvation.
However, the hydroxyl groups tend to be present on the particle surfaces in a non-uniform manner. Therefore, when particle bombardment due to thermal motion is repeated, particles easily aggregate due to the non-uniform presence of hydroxyl groups. Thus, in addition to aggregation between the nickel oxide and stabilized zirconia, electrostatic aggregation occurs among particles of the same material (nickel oxide particles, or stabilized zirconia particles), leading to insufficient nickel dispersibility in the obtained composite oxide.
Further, since the true density of the nickel oxide and stabilized zirconia is so higher than that of the solvent, separation due to particle sedimentation is likely to occur. In addition, since the particle size is enlarged due to electrostatic aggregation, the separation becomes more significant, leading to insufficient nickel dispersibility in the obtained composite oxide.
Patent Literature 3 discloses an attempt to micronize nickel and uniformly disperse nickel oxide with a spray pyrolysis method using an aqueous solution obtained by dissolving a water-soluble nickel salt and a water-soluble zirconium salt.
However, in this technique, there is a significant difference in the precipitation rate between the nickel salt and the zirconium salt resulting from the difference in their solubility in the solvent. This causes a problem in that the nickel oxide is not dispersed uniformly in the composite oxide. In addition, depending on the types of water-soluble nickel salt and water-soluble zirconium salt, a large amount of acid gas is generated upon thermal decomposition, which makes it difficult to manufacture the composite oxide.